1. Field of the Invention
This invention relates to a microporous film, and more particularly to a microporous film comprising a matrix composed of a polyolefin and an inorganic filler and void spaces formed therein and having overall a microporous structure.
2. Description of the Prior Art
There have heretofore been proposed:
(1) a porous film prepared by the method kneading and molding a polyvinyl chloride resin, a solvent for the resin, a plasticizer and silica into a film, and then subjecting the molded film to drying process (See, for example Japanese Patent Application Publication No. 2922/1962);
(2) a porous film prepared by sintering a mixture of powder polyvinyl chloride resin and finely divided silica (See, for example Japanese Patent Application Publication No. 3092/1960); and;
(3) a porous film prepared by molding a mixture of a polyolefin having a standard load melt index of 0 and a viscosity average molecular weight of 300,000 or more, silica and a plasticizer into a film and extracting the plasticizer from the molded film. (See, for example U.S. Pat. No. 3,351,495).
In case a porous film is employed as a separator in a lead accumulator, the film is required to have a small electrical resistance in the electrolyte. For a high-performance separator, the film should have an electrical resistance lower than 0.0006 .OMEGA.dm.sup.2 /sheet. To meet this requirement, the film should have high void space rate or porosity to enjoy an electrical resistance as low as 0.0003 .OMEGA.dm.sup.2 /0.1 mm (in thickness) or less and a thickness of about 0.2 mm. In case the electrical resistance is higher than 0.0003 .OMEGA.dm.sup.2 /0.1 mm, the film should be formed extremely thin, to wit, less than 0.2 mm thick.
In this connection, it is to be noted that the conventional porous films (1) and (2) made of a polyvinyl chloride resin as mentioned above can be imparted with an electrical resistance as low as 0.0003 .OMEGA.dm.sup.2 /0.1 mm, but they have such defects as being brittle and lacking flexibility so that it is extremely difficult to form them into a film having a thickness of less than 0.4 mm which is suitable for practical use. On the other hand, the conventionally proposed porous film made of an ordinary polyolefin and a vast amount of inorganic filler fails to have flexibility and is too brittle to be practically utilizable. The method mentioned above with reference to said film (3) has been proposed to solve the problem of brittleness and lack of flexibility. Said method is not satisfactory because the super-high molecular weight polyolefin employed (that having a standard load melt index of zero and a viscosity average molecular weight of 300,000 (which is nearly equal to a weight average molecular weight)) has low flow characteristics. Further, said film (3) has an electrical resistance higher than 0.0003 .OMEGA.dm.sup.2 /0.1 mm. Thus, it is difficult to produce a high-performance separator having an electrical resistance as low as 0.0006 .OMEGA.dm.sup.2 /sheet or less from the conventional films.